Electric discharge lamp



ELECTRIC DISCHARGE LAMP Filed May 15, 1936 Inventor: Henry G Jenkins,

y fi itorney v Patented July 9, 1940 UNITED STATES memo mscnanen mm Henry Grainger Jenkins, Pinner, England, 'assisnor to General Electric Company, a corporation of New York Application May 15, 1936, Serial No. 79,868 In Great Britain May 30, 1935 3 Claims.

This invention relates to electric lamps of the type comprising a vessel containing matter in the gaseous state which carries an electric discharge and, within the said vessel, a layer of ma- 5 terial adapted to be excited by the discharge to luminescence (sometimes loosely called fluorescence) It relates more particularly to lamps of this type with cold electrodes used for advertising and run at voltages greatly exceeding ordinary distribution voltages.

Heretofore lamps of this type have contained mercury vapour and the radiation emitted from the discharge has been mainly the mercury spectrum; if rare gases have been added to the mer- 15 cury in order to facilitate starting, their spectrum has not been emitted to any appreciable extent in operation. Now the mercury spectrum isrich in radiation having wave-lengths between the limits 1800 to 4000 Angstroms (which will go be termed the near ultra-violet) the luminescent materials have therefore been such as are excited powerfully by such radiation,

However it is not certain that, even in such mercury lamps, near ultra-violet radiation is the 25 only agency exciting the materials used to luminescence. There may have been some excitation by radiation of wave-length shorter than 1800 Angstroms (which will be termed far ultraviolet), by metastable atoms, or by slow elec- 3 trons. For all these agencies are present to some extent in the mercury discharge, and, when the luminescent material is within the envelope, it is not easy to determine experimentally which of them is efiective. By slow electrons we mean 35 those having an energy of a few electron-volts,

such as are present in large numbers in the positive column of an electric discharge. Here it may be observed that the invention does not include any discharge devices in which luminescent material is excited mainly or wholly by fast electrons, having an energy of hundreds of thousands of electron-volts, such as carry the current in a cathode ray tube; even if a cathode ray tube contains a little gas to concentrate the beam, the

gas does not carry the discharge in the sense relevant to the present invention.

I have found that marked advantages may be obtained by replacing mercury vapour by other gases which, when they carry the discharge, emit 5o much less near ultra-violet radiation in proportion to visible radiation, but emit a considerable proportion of far ultra-violet radiation. Rare gases, and particularly neon, are suitable. The luminescent material associated with the dis- 55 charge must then, of course, be one powerfully excited by far ultra-violet radiation or by the other agencies mentioned which accompany it. In the case of neon the strong line at about 740 Angstroms may be very effective in exciting luminescence. 5 One of the advantages of the substitution of these gases for mercury is a very great increase in the length of life of eiiicient operation of the luminescent materials. Another advantage, possessed particularlyby neon, is the fact that the primary visible light from the discharge can be used to greater advantage when combined with the secondary light from the luminescent material than can the visible light from. mercury.

My invention, therefore, comprises an electric lamp provided with a gaseous filling which, when it carries a discharge, emits much less near ultra-violet radiation in relation to visible radiation than does mercury, but emits a substantial o amount of far ultra-violet radiation, and provided also with material within the envelope of the discharge which will emit visible luminescent radiation when in contact with such a gaseous filling when it is carrying a discharge.

Suitable materials which thus emit visible luminescent radiation are zinc silicate and calcium tungstate. The zinc silicate may be of the wellknown variety, known as willemite, which emits green luminescent light, or of the less known variety, described in co-pending application Serial Number 79,761, filed May 14, 1936, which emits yellow luminescent light. Calcium tungstate emits a bluish luminescent light. For a gaseous filling in lamps made according to my invention, neon is considered the most suitable, be-

cause the orange light emitted from a discharge from neon can be most advantageously combined with the light from the luminescent material. Other rare gases, and in particular helium, may also be used in discharge lamps provided with luminescent material in accordance with my invention.

Lamps constructed according to my invention need not difier substantially from the known mercury lamps, except in the nature of the gaseous filling. The accompanying drawing shows somewhat conventionally an electric lamp embodying the present invention. In the drawing, a tubular glass envelope l is filled with neon at a pressure of about 5 mm.; it is provided at either end with an electrode 2, 3 of sheet metal; the interior surface of the envelope is coated with a thin layer 4 of zinc silicate or calcium tungstate or of a mixture of these two materials.v

- minous efficiency. the current density of the discharge must be chosen suitably. Thus if the discharge tube is of the diameter usual in lamps of this type (10-25 mm.) and is filled with neon to the usual pressure (2-10 mm. of mercury), then the e'fllciency will generally increase at first with the current density; but if a certain limit is exceeded, though the total emciency may continueto increase, the ratio of luminescent todirect radiation will decrease. In all such tubes this limiting current density was less than 0.23 ampere per sq. cm. (A/sq. cm). When the dimeter was 11 mm. and the pressure 5 mm., the

optimum current density was about 0.04 A/sq.

cm.; when the diameter was 20 mm. and thepressure 5 mm., the optimum current density was about 0.023 A/sq. cm. The luminescent material to which these values refer was willemite.

If a coating of willemite is used in accordance with my invention, the orange light generated by the neon discharge combines with the green light produced by the luminescence of the willemite to produce a bright yellow light of high efllciency. If calcium tungstate is used alone, a light of agreeable pink color can be obtained by the combination of the light from the neon and the light from the tungstate, If both wlllemite and calcium tungstate are used, the resultant light may be varied from yellow through yellowish white to purplish white, depending on the percentages of the two materials which are present.

In accordance with one embodiment of my in,- ventiona mixture of 40 to 50 per cent calcium tungstate and 50 to 60 per cent willemite when applied in a lamp such as shown inthe drawing and comprising a tubular container about 15 mm. in diameter and being fllled with neon gas at a pressure of about 5 mm. of mercury resulted in a light unit capable when operating of emitting approximately white light with an emciency of 25 lumens per watt. The most favorable proportions of these ingredients will vary considerably with the method of preparation of the luminescent materialst For example, in the aforesaid illustrative embodiment, when a different sample of willemite was used, having much smaller particles, the best proportions were found to be 20 per cent of willemite and 80 per cent of calcium tungstate,

Lamps made in accordance with my invention will not only have a high luminous efilciency and a commercially long life but are of particular value for the purpose of giving novel color effects in lamps used for advertising and display purposes.

I claim:

1. An electric lamp comprising an envelope, cooperating electrodes located respectively adjacent to the ends of said envelope, a filling of neon at a pressure between 2 and mm. of mercury for the envelope to produce a substantial amount ofvisible radiation when excited by the discharge between said electrodes, and a layer on the interior surface of the envelope consisting of a mixture of luminescent zinc silicate and calcium tungstate which is powerfully excited by the far ultra violet radiation present in neon when it is excited by the discharge between said electrodes to emit visible luminescent radiation which substantially supplements the visible radiation from said'dlscharge, the luminescent material, when excited by the discharge, emitting visible radiations of such intensity relative to the absorptive capacity of said material for visible radiations that the light efficiency of the lamp emitting the combined radiations is at least equal to that of the lamp if the luminescent material were absent.

2. An electric lamp comprising an envelope,

' cooperating electrodes located respectively adjacent to the ends of said envelope, a luminosity producing filling of neon at a pressure between 2 and 10 mm. of mercury for the envelope to produce a substantial amount of visible radiation when excited by the discharge between said electrodes, and a layer on the interior surface of the envelope consisting of luminescent zinc silicate which is powerfully excited by the far ultra violet radiation present in neon when it'is excited by the discharge between said electrodes to emit visible luminescent radiation which substantially supplements the visible radiation from said discharge, the luminescent material, when excited by the discharge, emitting visible radiations of such intensity relative to the absorptive capacity of said material for visible radiations that the light efilciency of the lamp emitting the combined radiations is at least equal to that of the lamp if the luminescent material were absent.

3. An electric lamp comprising an envelope, cooperating electrodes located respectively adjacent to the ends of said envelope, a luminosity producing filling of neon at a pressure between 2 and 10 mm. of mercury for the envelope to produce a substantial amount of visible radiation when excited by the discharge between said electrodes, and a layer on the interior surface of the envelope consisting of luminescent calcium tungstate which is powerfully excited by the far ultra violet radiation present in neon when it is excited by the discharge between said electrodes to emit visible luminescent radiation which substantially supplements the visible radiation from said discharge, the luminescent material, when excited by the discharge, emitting visible radiations of such intensity relative to the absorptive capacity of said material for visible radiations that the light efhciency of the lamp emitting the combined radiations is at least equal to that of the lamp if the luminescent material were absent.

HENRY GRAINGER JENKINS. 

